PWM controller with temperature regulation of switching frequency

ABSTRACT

A circuit for controlling the switching frequency of an oscillator for a PWM controller for a switching mode power supply, comprising a first stage providing a signal which is related to the temperature of the controller for the switching mode power supply; and an oscillator having an oscillation frequency and being responsive to the signal from the first stage for changing the oscillation frequency such that the oscillation frequency decreases as the temperature increases and vice versa.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims the benefit and priority of U.S.Provisional Application Ser. No. 60/602,477 filed Aug. 18, 2004 entitledTEMPERATURE REGULATION IN PWM CONTROLLER, the entire disclosure of whichis incorporated by reference herein.

BACKGROUND OF THE INVENTION

The present invention relates to switching mode power supplies, and inparticular, to switching mode power supply controllers utilizing pulsewidth modulation to control the on times of the power transistor outputstages. In such switching mode PWM controllers, the designs are usinghigher switching frequencies to take advantage of the reducedinductance/magnetic sizes that can be used at these frequencies. A majordisadvantage or operating at higher switching frequencies is theincreased switching and gate drive losses, resulting in lower systemefficiency and increased board heat, that is, a rise in temperature. Incertain applications, this increase in temperature cannot be tolerateddue to reliability and system integrity concerns.

In the prior art, it is known that some PWM control ICs have a featurewhereby the controller enters a hysteretic or “light load” mode. Thecontroller enters this mode under low output current conditions, and theregulator only delivers power to maintain the output voltage.Consequently, the switching frequency can vary. This method is intendedto reduce power loss to extend battery life in battery-poweredapplications. However, this prior art circuit does not provide forreduction in switching frequency as the temperature of the circuitincreases. Further, it applies to a hysteretic mode converter wherebythe power transistor turns on only to maintain the output voltage. Giventhis control, the frequency can vary depending on the output load.According to the present invention, it is desired to provide acontroller which operates in a pulse width modulation mode whereby theswitching frequency is maintained in a constant frequency operation butwill change as a function of temperature changes. Accordingly, incontrast to the hysteretic converters wherein the switching frequencyconstantly changes as a function of output load, the present inventionchanges the switching frequency based on temperature change duringsteady state PWM operation.

SUMMARY OF THE INVENTION

According to the invention, a PWM control circuit is provided to adjustthe switching frequency of the switching regulator in order to regulatethe temperature of the circuit and maintain system integrity. Thecircuit may be an integrated circuit. This circuit comprises thefollowing:

A temperature sensing circuit which uses the temperature dependence of asemiconductor device, for example, a bipolar transistor, to create acurrent source which is proportional to absolute temperature.

An amplifier circuit which acts as a voltage controlled current source.The current proportional to the absolute temperature is fed to aresistor to create a voltage proportional to the absolute temperature.This voltage is fed to a first input of the amplifier. The second inputis tied to a nominal reference voltage. The output of this circuit is acurrent which is inversely proportional to temperature.

An oscillator circuit which uses the current inversely proportional totemperature and a capacitor to set the switching frequency.

With this circuit, as the temperature in the system increases, theswitching frequency will consequently decrease, therefore reducing powerloss and reducing the temperature thereby providing a temperatureregulation circuit for the PWM switching controller.

In accordance with the above, the invention comprises a circuit forcontrolling the switching frequency of an oscillator for a PWMcontroller for a switching mode power supply, comprising a first stageproviding a signal which is related to the temperature of the controllerfor the switching mode power supply; and an oscillator having anoscillation frequency and being responsive to the signal from the firststage for changing the oscillation frequency such that the oscillationfrequency decreases as the temperature increases and vice versa.

Other objects, features and advantages of the present invention willbecome apparent from the detail description which follows.

BRIEF DESCRIPTION OF THE DRAWING(S)

The invention will now be described in greater detail in the followingdetailed description with reference to the drawings in which FIG. 1shows a circuit diagram of the circuit according to the invention forregulating the switching frequency of the oscillator controlling the PWMswitching frequency in accordance with temperature.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

With reference now to the drawing figure, FIG. 1 shows a circuitaccording to the invention. The circuit comprises three portions: atemperature sensing circuit 10, an amplifier 20 and an oscillator 30.

In the illustrated embodiment, the temperature sensing circuit is made apart of the switching controller so that it responds to the temperatureof the switching controller circuit. The switching controller circuitmay be disposed in an integrated circuit package and will control theoutput stage of a switching mode power supply which may comprise atleast one power output transistor such as a power MOSFET. This is notshown. However, the power MOSFET will have its gate controlled by apulse width modulated signal from a controller whose switching frequencyis controlled by the oscillator circuit according to the presentinvention. The on time of the output transistor is pulse width modulatedto control the output voltage. In many applications, the output stagewill comprise two output transistors connected across a DC bus voltagein a boost or buck configuration to boost or reduce the DC bus voltageto the required voltage. However, the switching mode power supply outputstage can be any type of switching mode output stage other than a buckor boost stage.

With reference to FIG. 1, the temperature sensing stage comprises, inthe illustrated embodiment, bipolar transistors Q1, Q2, Q3 and Q4, inthe illustrated embodiment, NPN bipolar transistors, which are coupledbetween a voltage source VCC and ground. In the current generatorcircuit, Q3 and Q4 force the current in each leg to be equal. Q2 issized for example, 8 times larger than Q1 which creates a difference inVbe voltage of Q1 (Vbe1) and Q2 (Vbe2). This difference, DeltaVbe, showsup across resistor R2 and sets the current (IPTAT) which is nowproportional to absolute temperature. This current is mirrored by thecurrent mirror of M1-M2 and creates a voltage proportional to absolutetemperature across resistor R3. This voltage feeds into the GM amplifierA which then adjusts the oscillator current to regulate the converterfrequency.

The voltage across resistor R3 proportional to IPTAT is coupled to theinverting input of the transconductance amplifier A. The non-invertinginput of the amplifier A is coupled to a nominal reference voltage V1.The output of the amplifier A is a current inversely proportional toabsolute temperature (IIPTAT) and charges a capacitor C1 of anoscillator 30. When the voltage across the capacitor C1 increases abovethe threshold V2 of the comparator COMP, the comparator switches highand a voltage is provided to two legs of the oscillator circuit. In oneleg comprising inverters U6A and U7A which provide a delayed pulseacross a capacitor C2. The undelayed signal and the delayed signal arecoupled to an AND gate U9A and the delayed rising edge is coupled to afurther inverter stage U8A which provides a falling edge. The undelayedrising edge and the falling edge from inverter U8A are coupled to afurther AND gate U10A to provide a pulse output. The pulse output isprovided back to the gate of a MOSFET M3 to discharge the capacitorafter the delay time which allows the capacitor to again be charged bythe current IIPTAT to restart the oscillator cycle. Depending upon thevalue of the current IIPTAT, the capacitor will be charged at a rateinversely proportional to the absolute temperature. Thus, if thetemperature increases, the capacitor will take longer to charge, therebyreducing the switching frequency. If the absolute temperature decreases,the current IIPTAT will increase, thus allowing the capacitor to chargemore quickly, decreasing the switching frequency. The oscillator outputis shown at OSCOUT.

Accordingly, as the temperature increases, the switching frequency isdecreased to reduce switching losses.

The present invention provides a number of advantages. It preventssystem overheating while still delivering full load current at thespecified output voltage. The reduced switching frequency is maintainedin a constant frequency operation (steady state) mode, which does notintroduce unwanted noise into the system. The invention can takeadvantage of allowing smaller magnetic/inductive components at higherfrequencies without the concern of system overheating.

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art.Therefore the present invention should be limited not by the specificdisclosure herein, but only by the appended claims.

1. A circuit for controlling the switching frequency of an oscillatorfor a PWM controller for a switching mode power supply, comprising: afirst stage providing a signal which is related to the temperature ofthe controller for the switching mode power supply; and an oscillatorhaving an oscillation frequency and being responsive to the signal fromthe first stage for changing the oscillation frequency such that theoscillation frequency decreases as the temperature increases and theoscillation frequency increases as the temperature decreases.
 2. Thecircuit of claim 1, wherein the first stage comprises: a first circuitresponsive to the temperature of the controller and providing a currentproportional to the absolute temperature of the controller; and a secondcircuit responsive to the current proportional to the absolutetemperature from the first circuit and providing a second currentinversely proportional to the absolute temperature and wherein thecurrent inversely proportional to the absolute temperature is providedto the oscillator circuit such that as the temperature increases, theoscillation frequency of the oscillator reduces.
 3. The circuit of claim2, wherein the first circuit comprises: at least one bipolar transistorresponsive to the absolute temperature of the controller; and whereinthe second circuit comprises an amplifier stage having an invertinginput and a non-inverting input with said current proportional to theabsolute temperature being fed through a resistor coupled to saidnon-inverting input to provide a voltage proportional to the absolutetemperature, and whereby said amplifier produces an output comprisingsaid current inversely proportional to the absolute temperature.
 4. Thecircuit of claim 3, wherein said current inversely proportional to theabsolute temperature charges a timing capacitor, said timing capacitorbeing coupled to an input of a comparator, the comparator providing anoutput when the voltage across said capacitor exceeds a predeterminedreference voltage coupled to said comparator.
 5. The circuit of claim 4,wherein the oscillator circuit comprises at least one delay stage and afeedback line for discharging said capacitor after a delay timeimplemented by said delay stage.
 6. The circuit of claim 5, wherein theoscillator comprises a controlled switch controlled by said feedbackline for discharging said capacitor.
 7. A method for controlling theswitching frequency of an oscillator for a PWM controller for aswitching mode power supply, comprising: providing a first signal whichis related to the temperature of the controller for the switching modepower supply; and controlling an oscillator having an oscillationfrequency in response to the first signal by changing the oscillationfrequency such that the oscillation frequency reduces as the temperatureincreases and as the temperature decreases, the oscillator frequency ofthe oscillator increases.
 8. The method of claim 7, wherein the step ofproviding a first signal comprises: providing a current proportional tothe absolute temperature of the controller; and responsive to thecurrent proportional to the absolute temperature, providing a secondcurrent inversely proportional to the absolute temperature and providingthe current inversely proportional to the absolute temperature to theoscillator such that as the temperature increases, the oscillationfrequency of the oscillator decreases.
 9. The method of claim 8, whereinthe step of providing a current proportional to the absolute temperaturecomprises: providing at least one bipolar transistor responsive to theabsolute temperature of the controller; and providing an amplifier stagehaving an inverting input and a non-inverting input with said currentproportional to the absolute temperature being fed through a resistorcoupled to said non-inverting input to provide a voltage proportional tothe absolute temperature, and whereby said amplifier produces an outputcomprising said current inversely proportional to the absolutetemperature.
 10. The method of claim 9, further comprising charging atiming capacitor with said current inversely proportional to theabsolute temperature, and providing a voltage across said timingcapacitor to an input of a comparator for providing an output when thevoltage across said capacitor exceeds a predetermined reference voltagecoupled to said comparator.